Conventionally, the compressor impeller of a turbo machine such as a turbocharger, is fixed to a drive shaft provided integrally with a turbine by means of a nut. That is, the compressor impeller has an axially extending through-hole, into which the drive shaft is inserted, and the nut is threadedly engaged with a screw portion provided on the distal end portion of the drive shaft, and fastened for fixation.
However, in such a connection structure, in which the compressor impeller is provided with a through-hole, high stress is liable to be generated at some midpoint in the axial direction thereof, and there is a limit to an improvement in terms of durability.
In view of this, there has been proposed a connection structure in which the compressor impeller is provided with, in stead of a through-hole, a bottomed screw hole (blind hole) extending in the axial direction, and in which the drive shaft is threadedly engaged with this screw hole (See, for example, Patent Document 1 and Patent Document 2). Further, in Patent Document 1 and Patent Document 2, there is provided a fit-engagement portion realized through interference fit of the drive shaft with the hole in order to eliminate back-lash in the threaded-engagement portion and to achieve an improvement in terms of concentricity.
In this connection structure, it is only necessary for the screw hole to be long enough to ensure reliable threaded engagement of the drive shaft, and there is no need for it to extend to a midpoint in the axial direction, so that generation of high stress is not easily involved.
[Patent Document 1] 05-504178 A
[Patent Document 2] U.S. Pat. No. 5,193,989